The present invention is related to a process for preparing lithium manganate in spinel structure, and more particularly to a process for preparing lithium manganate in spine structure, which can suppress the dissolving amount of the manganese therefrom when said lithium manganate is used as an anode material for nonaqueous electrolyte secondary battery and can improve the high temperature property of the secondary battery, such as storage property under high temperature and cycle property at high temperature.
Due to recent tendency of popularization of portable computers, telephones, etc. and their change to be cordless, demand for secondary batteries as a drive power has been increased. Particularly, nonaqueous electrolyte secondary battery is the most expected battery since it is small in the size and has high energy density. As an anode material to be used for the nonaqueous electrolyte secondary battery which can meet to the requirement as described above, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate and the like can be given as the examples. Since these complex oxides generate higher voltage than lithium as much as 4V or more, they can be useful as a battery generating higher energy density.
Out of the complex oxides described above, the theoretical capacity of LiCoO2 and LiNiO2 is more or less 280 mAh/g.
However, the theoretical capacity of LiMn2O4 is rather small as much as 148 mAh/g, but manganese oxide compounds as the raw material are rich in the availability and cheap, and it has no thermal instability at the time of charging contrary to the case of LiNiO2, and therefore, it seems to be suitable as an anode material for EV use.
However, lithium manganate (LiMn2O4) in spinel structure gives dissolution of manganese at high temperature range, and it has therefore a problem that the battery property at a high temperature of the lithium manganate in spinel structure, such as storage property and cycle property at a high temperature, is not sufficient.
Therefore, it is an object of the present invention to provide a process for preparing lithium manganate in spinel structure which can suppress the dissolving amount of manganese at charging when it is used as the anode material for nonaqueous electrolyte secondary battery, of which battery property at high temperature range, such as storage property and cycle property at a high temperature, is improved, and to provide an anode material composed of the lithium manganate in spinel structure and nonaqueous electrolyte secondary batteries using the said anode material.
Improvement in the battery property of the lithium manganate in spinel structure at a high temperature has been tried by substituting the part of manganese or lithium elements with various other elements. The lithium manganate in spinel structure is normally obtained by adding a compound, which contains an element to be used for the substitution, to raw manganese material or raw lithium material and mixing, and then burning the resulting mixture. Whereas, the electrolyzed manganese dioxide is an appropriate raw manganese material for the lithium manganate in spinel structure since it is rich in market availability and is cheap.
Normally, following to electrolysis, the electrolyzed manganese dioxide is subjected to neutralization with ammonia when it is used for manganese dry battery, besides it is subjected to neutralization with caustic soda when it is used for alkaline manganese battery. It is known that a small amount of sodium remains in the electrolyzed manganese dioxide when the electrolyzed manganese dioxide is neutralized with caustic soda, and the amount of remaining sodium depends on the neutralization condition. Similarly, when using potassium instead of sodium for the neutralization, small amount of potassium still remains in the electrolyzed manganese dioxide, and the amount of the potassium depends on the neutralization condition as well.
The inventors of the present invention found that the lithium manganate in spinel structure suitable to achieve the object as described above can be obtained by specifically establishing a neutralization condition for the electrolyzed manganese dioxide and suitable elements to substitute.
The process for preparing the lithium manganate in spinel structure of the present invention is established based on the finding described above, and the process is characterized as constituted of a step to neutralize the electrodeposited manganese dioxide with either a sodium compound or a potassium compound, a step to add a raw lithium material and a compound containing at least one element selected from a group consisting of aluminum, magnesium, calcium, titanium, vanadium, chromium, iron, cobalt, nickel, copper and zinc for substituting the part of the manganese contained in the lithium manganate by at least one element selected from the group described above to the neutralized electrolyzed manganese dioxide and a step to mix and burn the resulting mixture.
The invention is characterized in that the sodium compound or the potassium compound is either a hydroxide compound or a carbonate compound.
The invention is characterized in that the amount of the manganese of which part being substituted by at least one element selected from a group consisting of aluminum, magnesium, calcium, titanium, vanadium, chromium, iron, cobalt, nickel, copper and zinc described in claim 1 is specified to a range of from 0.5 to 15 mol %.
The invention is characterized in that the burning operation in the processes is respectively carried out at a temperature higher than 750xc2x0 C.
The invention of the anode material for nonaqueous electrolyte secondary battery use is characterized in that the anode material is composed of the lithium manganate in spinel structure obtained according to any of the processes.
The invention is characterized in that the inventive nonaqueous electrolyte secondary battery is constituted of an anode composed of the anode material, an cathode composed of lithium, a lithium alloy and a material capable of occluding and deoccluding lithium and a nonaqueous electrolyte.